1. Technical Field
The present invention relates to an optical scanning apparatus, an image-reading apparatus and an image-forming apparatus, in all of which the straightness of the caustic curve is stably maintained irrespective of the temperature changes, thereby keeping the bending of the scanning line to a minimum.
2. Related Art
In an image-forming apparatus using the Chester Carlson's process, there are performed the formation of latent images, development of the images and transfer of the images according to the rotation of a photoreceptor drum. In a multi-color image-forming apparatus in which a plurality of photoreceptor drums are disposed in a direction of transporting the medium on which an image is to be transferred, and in which the toner image formed in each color in each of the image-forming stations is overlapped with one other, there are the following disadvantages. In other words, due to the eccentricity or variation in diameter of each photoreceptor drum, there occurred deviations 1) in time from the latent image formation to the transferring, and 2) in the parallelism among the shafts of the respective photoreceptor drums of respective colors. Another disadvantage is that, due to 3) variation in the velocity of, or skew of, the medium on which the image is to be transferred, e.g., a transportation belt which transports the recording paper, there will occur resist deviation in the sub-scanning direction of each of the toner images, resulting in deterioration of image quality.
Similarly, also in an optical scanning apparatus, unless the writing position of the electrostatic latent image to be formed on the photoreceptor drum, and the parallelism among the scanning lines for scanning each of the photoreceptor drums are accurately aligned, there will occur a resist deviation or skew, resulting in color deviations or color changes.
Generally, the optical scanning apparatus is made up of: a light source having a semiconductor laser which is modulated by a predetermined pixel clock based on pixel data of the recording image; a polygon mirror which radially scans through deflection the light beams from the light source; and an optical scanning system having an fθ characteristic which forms the scanned optical beam into an image in spots on the surface of the photoreceptor drum and which arrays so that the distances among the adjacent spots of pixels become uniform. In order to reduce the cost and to secure high degree of freedom in the surface shape, the scanning lenses constituting the optical scanning system are nowadays made mostly of resin.
In this manner, the resin-formed optical system has a problem in that the rigidity of the system is low and therefore that deformation thereof is likely to take place due to stresses and temperature changes. As a solution, JP-A-2006-215397 discloses an art to use a supporting plate having a pair of receiving parts which are larger in rigidity than the optical element and which support the optical system, in the sub-scanning direction, near both end portions, as seen in the main scanning direction, of the optical system.
However, in the above-described arrangement of the prior art, if the free expansion of the optical element is not hampered, the straightness of the caustic curve can be stably maintained, thereby preventing the occurrence of bending of the scanning line. It is to be noted, however, that the rigid body member made of metallic plate work and the optical element made of resin have different coefficients of thermal expansion. Therefore, in case of a temperature change after initial adjustment or calibration, the adjusting screws, engagement parts for positioning, and pressurizing means will hamper or prevent free thermal expansion thereof. In this manner, the straightness of the caustic curve is deteriorated, thereby giving rise to bending of the scanning line.